Phase Space Cuts

PhaseSpace is base class for all hard-process phase-space generators, either generic 2 -> 1 or 2 -> 2 ones, or specialized ones like for elastic and diffractive scattering.

In it, it is possible to constrain the kinematics of most processes. (Exceptions are "soft physics", i.e. minimum bias, elastic and diffractive processes. The Coulomb singularity for elastic scatterings, if simulated, is handled separately.) These constraints apply in the rest frame of the hard subprocess, and topologies normally would be changed e.g. by subsequent showering activity. The cross section of a process is adjusted to only correspond to the allowed phase space.

The more particles in the final state, the more cuts could be applied. Here we have tried to remain with the useful minimum, however. More generic possibilities could be handled by the user hooks facility.

Cuts in all processes

parm  PhaseSpace:mHatMin   (default = 4.; minimum = 0.)
The minimum invariant mass.

parm  PhaseSpace:mHatMax   (default = -1.)
The maximum invariant mass. A value below mHatMin means there is no upper limit.

Cuts in 2 -> 1 processes

When a resonance id is produced, the mMin(id) and mMax(id) methods restrict the allowed mass range of this resonance. Therefore the allowed range is chosen to be the overlap of this range and the mHatMin to mHatMax range above. Most resonances by default have no upper mass limit, so effects mainly concern the lower limit. Should there be no overlap between the two ranges then the process will be switched off.

Cuts in 2 -> 2 processes

parm  PhaseSpace:pTHatMin   (default = 0.; minimum = 0.)
The minimum invariant pT.

parm  PhaseSpace:pTHatMax   (default = -1.)
The maximum invariant pT. A value below pTHatMin means there is no upper limit.

parm  PhaseSpace:pTHatMinDiverge   (default = 1.; minimum = 0.5)
Extra pT cut to avoid the divergences of some processes in the limit pT -> 0. Specifically, if either or both produced particles have a mass below pTHatMinDiverge then pT is limited from below by the larger of pTHatMin and pTHatMinDiverge.

flag  PhaseSpace:useBreitWigners   (default = on)
Allows masses to be selected according to Breit-Wigner shapes in 2 -> 2 processes, whenever particles have been declared with a nonvanishing width above the threshold below. In those cases also the limits below will be used for the mass selection. For 2 -> 1 processes the Breit-Wigner shape is part of the cross section itself, and therefore always included.

parm  PhaseSpace:minWidthBreitWigners   (default = 0.01; minimum = 1e-6)
The minimum width a resonance must have for the mass to be dynamically selected according to a Breit-Wigner shape, within the limits set below. Only applies when useBreitWigners is on; else the nominal mass value is always used.

For a particle with a Breit-Wigner shape selected, according to the rules above and to the rules of the particle species itself, the mMin(id) and mMax(id) methods restrict the allowed mass range of the particle, just like for the 2 -> 1 processes.

Cuts in 2 -> 3 processes

Currently no further cuts have been introduced for 2 -> 3 processes than those already available for 2 -> 2 ones. A 2 -> 3 process in principle would allow to define a separate pT range for each of the three particles (with some correlations), but for now all three are restricted exactly the same way by pTHatMin and pTHatMax. As above, Breit-Wigner mass ranges can be restricted.

Cuts for a second hard process

If you use the machinery that allows the generation of a specified second hard process then, by default, the same phase space cuts will be used for it as listed above. Optionally, however, you may use a second set of cuts, as described here. In this context "first" and "second" is merely a technical distinction; you are welcome e.g. to pick pT ranges such that the second interaction always has a larger pT than the first.

flag  PhaseSpace:sameForSecond   (default = on)
By default use the same cuts for a second hard process as for the first. If off then instead use the mass and pT cuts below, where relevant. (The other cuts above still remain the same.)

parm  PhaseSpace:mHatMinSecond   (default = 4.; minimum = 0.)
The minimum invariant mass for a second interaction, if separate.

parm  PhaseSpace:mHatMaxSecond   (default = -1.)
The maximum invariant mass for a second interaction, if separate. A value below mHatMin means there is no upper limit.

parm  PhaseSpace:pTHatMinSecond   (default = 0.; minimum = 0.)
The minimum invariant pT for a second interaction, if separate.

parm  PhaseSpace:pTHatMaxSecond   (default = -1.)
The maximum invariant pT for a second interaction, if separate. A value below pTHatMin means there is no upper limit.

Documentation

flag  PhaseSpace:showSearch   (default = off)
Possibility to print information on the search for phase-space coefficients that (in a multichannel approach) provides an analytical upper envelope of the differential cross section, and the corresponding upper estimate of the cross section. Of interest for crosschecks by expert users only.

flag  PhaseSpace:showViolation   (default = off)
Possibility to print information whenever the assumed maximum differential cross section of a process is violated, i.e. when the initial maximization procedure did not find the true maximum. Also, should negative cross sections occur, print whenever a more negative value is encountered.